The goal of this proposal is to take advantage of the structural symmetry of multimeric proteins, a rarely explored property, to arrive eventually at structurally complementary multidentate protein ligands with ultra-high affinity and specificity. The long term objective is to illuminate an area of fundamental biological interest: the molecular recognition properties of multidentate protein ligands and the use of such ligands to control protein functions. Specifically, this proposal encompasses the design, synthesis and evaluation of multidentate ligands targeting a pair of ideal model systems: the heat-labile enterotoxin from E. coli (LT) and the closely related cholera toxin secreted by V. cholerae (CT), which are both AB5 heterohexamers. The biological mechanism of the actions of LT and CT includes a critical step of receptor recognition on the target cell by the B pentamer. The five-fold symmetry of the B subunits of LT and CT offers good opportunities to develop pentadentate ligands with overall structures complementary to the arrangement of toxin receptor binding sites. Such ligands will be created stepwise using a modular approach with each module providing opportunities for further optimization Building on the principles of molecular recognition, our proposed work will combine the powers of combinatorial chemistry and structure-based design to arrive at ultrahigh affinity pentadentate ligands. The affinity of the ligands obtained will be investigated with a variety of analytical tools. Detailed thermodynamics of ligand-protein interaction will be studied using a series of mono- to penta-dentate ligands. The proposed research has broad implications for the field of molecular recognition in general since it is at the frontiers of investigations focusing on multidentate ligands interacting with multimeric proteins. In addition, high affinity ligands derived from our work have potential health benefits, as they may lead to the development of agents useful for the detection, treatment, and prevention of AB5 toxin-related enterotoxigenic diseases.